Ageing does not significantly affect performance in a spatial learning task in the domestic cat (Felis silvestris catus)

A spatial learning task was used to examine cognitive function in 36 cats (1.0–15.1 years of age), with a control for motor function. No significant age-related decline in cognitive function was found. Initial selection of 75 cats showed no significant age differences between young (0–3 years), adult (3.1–8 years), senior (8.1–12 years) and geriatric cats (12.1–15.1 years) locating food rewards in a holeboard box. Consequently, the senior and geriatric groups were combined because of a concern that age effects may not be seen in the 8–12-year-old cats. Differences between these three age groups of cats to learn the position of three food rewards in an array of 30 otherwise empty (food-scented) positions were non-significant. Differences between age groups remained non-significant for retrieval of rewards from these three positions even when masked with a tissue paper cover. There was no significant effect of age on speed of reaching each criterion. Although the total number of errors was similar, there was a significantly increased proportion of working memory errors (when the cat returned to a previously baited position) and decreased proportion of reference memory errors (when the cat put a nose or paw in an empty position) with increasing age. There was no effect of age on motor function, with success in a plank-crossing test related only to plank width. These findings support previous work suggesting that the cat differs from other species in the way in which it demonstrates age-related declines in cognitive function. Further studies using different testing methodologies are required to assess other cognitive functions that may decline with age.     

Cortisol release and heart rate variability in horses during road transport

Based on plasma cortisol concentrations it is widely accepted that transport is stressful to horses. So far, cortisol release during transport has not been evaluated in depth by non-invasive techniques such as analysis of salivary cortisol and faecal cortisol metabolites. Transport also causes changes in heart rate and heart rate variability (HRV). In this study, salivary cortisol, faecal cortisol metabolites, heart rate and HRV in horses transported by road for short (one and 3.5 h) and medium duration (8 h) were determined. With the onset of transport, salivary cortisol increased immediately but highest concentrations were measured towards the end of transport (4.1 ± 1.6, 4.5 ± 2.6, 6.5 ± 1.8 ng/ml in horses transported for one, 3.5 and 8 h, respectively). Faecal cortisol metabolite concentrations did not change during transport, but 1 day after transport for 3.5 and 8 h had increased significantly (p < 0.01), reflecting intestinal passage time. Compared to salivary cortisol, changes in faecal cortisol metabolites were less pronounced. Heart rate increased and beat-to-beat (RR) interval decreased (p < 0.05) with the onset of transport. Standard deviation of heart rate increased while root mean square of successive RR differences (RMSSD) decreased in horses transported for 3.5 (from 74 ± 5 to 45 ± 6 ms) and 8 h (from 89.7 ± 7 to 59 ± 7 ms), indicating a reduction in vagal tone. In conclusion, transport of horses over short and medium distances leads to increased cortisol release and changes in heart rate and HRV indicative of stress. The degree of these changes is related to the duration of transport. Salivary cortisol is a sensitive parameter to detect transient changes in cortisol release.     

A telemetry study on the diurnal distribution and activity of adult pikeperch, Sander lucioperca (L.), in a riverine environment

The diurnal movements and spatial distribution of adult pikeperch, Sander lucioperca, in the Elbe River, Czech Republic was observed using radio telemetry. The hypothesis that light intensity, within four different intervals (dawn, day, dusk, night), would determine the spatial distribution of pikeperch in a riverine environment were tested across a time span of 12 months. During the day, fish were located in deep water of the main channel, moving towards shallower waters during twilight and residing in the littoral zone, closest to the riverbanks, at night. Movement activity followed the behavioural pattern in a drainage canal with maximum at twilight and minimum at night. This suggests that nocturnal positions of adult pikeperch in the shallows were not associated with hunting but more likely with resting. Handling editor: J. Cambray     

Changes in cortisol release and heart rate and heart rate variability during the initial training of 3-year-old sport horses

Based on cortisol release, a variety of situations to which domestic horses are exposed have been classified as stressors but studies on the stress during equestrian training are limited. In the present study, Warmblood stallions (n = 9) and mares (n = 7) were followed through a 9 respective 12-week initial training program in order to determine potentially stressful training steps. Salivary cortisol concentrations, beat-to-beat (RR) interval and heart rate variability (HRV) were determined. The HRV variables standard deviation of the RR interval (SDRR), RMSSD (root mean square of successive RR differences) and the geometric means standard deviation 1 (SD1) and 2 (SD2) were calculated. Nearly each training unit was associated with an increase in salivary cortisol concentrations (p < 0.01). Cortisol release varied between training units and occasionally was more pronounced in mares than in stallions (p < 0.05). The RR interval decreased slightly in response to lunging before mounting of the rider. A pronounced decrease occurred when the rider was mounting, but before the horse showed physical activity (p < 0.001). The HRV variables SDRR, RMSSD and SD1 decreased in response to training and lowest values were reached during mounting of a rider (p < 0.001). Thereafter RR interval and HRV variables increased again. In contrast, SD2 increased with the beginning of lunging (p < 0.05) and no changes in response to mounting were detectable. In conclusion, initial training is a stressor for horses. The most pronounced reaction occurred in response to mounting by a rider, a situation resembling a potentially lethal threat under natural conditions.     

Towards a more objective assessment of equine personality using behavioural and physiological observations from performance test training

Current definitions of horse personality traits are rather vague, lacking clear, universally accepted guidelines for evaluation in performance tests. Therefore, the aim of the present study was to screen behavioural and physiological measurements taken during riding for potential links with scores the same horses received in the official stallion performance test for rideability and personality traits. Behaviour, heart rate (HR) and HR variability from thirty-six stallions participating in a performance test were recorded repeatedly during their performance test training. Using the coefficient of determination, regression analysis revealed that about 1/3 of variation (ranging between r = 0.26 (“constitution” (i.e. fitness, health)) and r = 0.46 (rideability)) in the personality trait scores could be explained by selecting the three most influential behaviour patterns per trait. These behaviour patterns included stumbling (with all traits except character), head-tossing (temperament, rideability), tail-swishing (willingness to work), involuntary change in gait (character) and the rider's use of her/his hands (constitution, rideability), voice (temperament) or whip (constitution). Subsequent mixed model analysis revealed a significant (P < 0.05) influence of the behaviour pattern “horse-induced change in gait” on character (−0.98 ± 0.31 scores per additional occurrence of change in gaits), of head-tossing (−0.25 ± 0.08 scores) and rider's use of voice (−0.51 ± 0.25; P = 0.0594) on temperament, and of stumbling on each of the following: willingness to work (−2.5 ± 1.2), constitution (−2.5 ± 1.2 scores; P = 0.0516) and rideability scores (−3.3 ± 1.4). In addition, constitution scores tended (P = 0.0889) to increase with higher low frequency/high frequency heart rate variation ratios (LF/HF), indicating a shift towards sympathetic dominance and thus a higher stress load in horses with higher scores for constitution. Rideability scores from the training phase were also significantly influenced by head-tossing (−0.5 ± 0.1), and in addition rideability scores from the final test were influenced by the training rider, ranging between average estimated rideability scores of 6.8 ± 0.4 for one training rider and 8.36 ± 0.3 scores for another training rider. Horses ridden with their nose-line predominantly behind the vertical received higher scores for rideability (8.3 ± 0.3) than horses ridden with their nose-line at the vertical (7.7 ± 0.2). These findings indicate that either judges perceive horses to have a better rideability when they readily offer a more extreme poll flexion, or that riders make use of horses’ better rideability by imposing a more extreme poll flexion. Several of the above described associations, but also of the non-existing links (e.g. no association between shying or heart rate and temperament) between behaviour patterns and scores for personality traits are rather surprising, warranting further investigation regarding the underlying causes of these relationships. Some of these behaviour patterns should be considered when redesigning the current guidelines for evaluation of personality traits during breeding horse performance tests, ultimately leading to improved genetic selection for equine personality traits. However, ethical implication of defining aversive behaviour such as head-tossing as an indicator of, for example, poor temperament, should not be neglected when devising new guidelines: such aversive behaviour may in fact be an indication of inadequate training techniques rather than poor horse personality.     

Intravenous administration of equine-derived whole IgG antivenom does not induce early adverse reactions in non-envenomed horses and cows

Administration of antivenoms to treat snakebite envenomings has the potential risk of inducing early adverse reactions. The mechanisms involved in these reactions are unclear. In this study, polyspecific antivenom consisting of whole IgG purified from equine plasma by caprylic acid precipitation was administered intravenously to non-envenomed horses (n = 47) and cows (n = 20) at a dose of 0.4 mL/kg. It has been reported that, in humans, this formulation (administered at a dose of 0.4 mL/kg) induces mild noticeable early adverse reactions, such as fever, vomiting, diarrhea, urticaria, generalized rash, tachypnea or tachycardia, in about 15–20% of the patients. Unexpectedly, none of the animals receiving antivenom in our study showed any evidence of early adverse reaction. Moreover, no late adverse reactions, i.e. serum sickness, were observed during 40 days after antivenom administration. Unlike studies performed in envenomed humans, our present results were obtained in a group of non-envenomed individuals. It is concluded that, in addition to the physicochemical characteristics of the formulation, other unknown factors must determine the occurrence of adverse reactions in snakebite envenomed humans treated with equine-derived antivenoms.     

Long-term telemetry of heart rates and energy metabolic rate during the diurnal cycle in normothermic and torpid African blue-naped mousebirds (Urocolius macrourus)

Colies are one of the phylogenetically oldest groups among the modern birds; the earliest finds are from about 35 million years ago. In states of energy deficiency they can undergo torpor during the night when metabolic rate and body temperature are decreased drastically to save energy (up to 90%). Here, we report the first measurements of heart rate (HR) by long-term telemetry, in seven individuals of blue-naped mousebirds (Urocolius macrourus); simultaneously and continuously metabolic rate (MR) was determined. HR at night was about 20% below the range of expected values (246/310 bpm). Mean oxygen pulse (O₂ output/stroke) in normothermic birds was in a range of 0.019–0.020 ml O₂/stroke; during torpor nights this value decreased significantly to 0.0086. Mean cardiac output ranged from 724 to 1214 ml blood/kg per min; in torpid birds this value fell to 400 ml blood/kg per min. Cardiac regulation of metabolic demand within an activity phase (day or night) is mainly achieved by chronotropy. Inotropy contributes at most 25% to the differences in MR between day and night (ca. 40%). Entry into torpor is brought about mainly by changes in HR (decrease from 240 to 90 bpm); after torpor levels have been reached, there is an increase in HR (to 200 bpm) and a sharp decrease (−53%) in stroke volume. This regulation by inotropy is also characteristic of arousal from torpor.     

Drosophila melanogaster does not share the telomeric repeat sequence of another invertebrate, Ascaris lumbricoides

Summary The DNA at the chromosomal termini of all eukaryotes from which it has been isolated contains a characteristic sequence motif consisting of tandem arrays of a regular or irregular repeat unit. These terminal repeats are thought to be essential for the maintenance of the chromosome ends. The sequences of the terminal repeats of all vertebrates studied thus far are identical and are similar enough to those of higher plants and some protozoans to cross-hybridize. However, previous studies have not detected cross-hybridization between the DNA of Drosophila mélanogaster and the terminal DNA sequences of any of several organisms tested. Recently, the first terminal DNA clone from a multicellular invertebrate, that of Ascaris lumbricoides, was reported also to consist of a tandem reiteration of a short sequence similar to those previously identified for other eukaryotes. Here I show that a probe for this sequence from A. lumbricoides fails to hybridize delectably to the DNA of D. melanogaster. Thus, in contrast to their conservation among vertebrates, the terminal chromosomal sequences appear not to be shared by all metazoan invertebrates.     

Clicker increases resistance to extinction but does not decrease training time of a simple operant task in domestic dogs (Canis familiaris)

Despite its popularity among pet owners and professional trainers, we are not aware of any studies that have investigated the efficacy of clicker training in canines. To this end, we taught 35 basenjis to nose-touch an orange traffic cone. Upon meeting pre-determined criteria, dogs progressed through: (1) training trials, wherein correct responses were followed immediately with a click plus food (clicker group) or food alone (control group); (2) strengthening trials, wherein dogs received the same reinforcement protocol as in training trials, except nose-touching behaviour was variably reinforced; and (3) extinction trials, wherein food was withheld from both groups, but dogs in the clicker group received a click alone for nose-touches. We found that the clicker and control groups did not differ with regard to the number of trials or the time required to meet training or strengthening criteria (P > 0.05 for all). However, the clicker group required significantly more trials (log₁₀ transformed means ± S.E. = 1.6 ± 0.03 trials versus 1.4 ± 0.03 trials, P < 0.001) and more time (log₁₀ transformed means ± S.E. = 2.85 ± 0.03 s versus 2.73 ± 0.03 s, P = 0.008) to reach extinction criterion. Additionally, younger dogs required fewer training (, P = 0.001) and strengthening (, P = 0.029) trials and less training (, P = 0.005) and strengthening (, P = 0.013) time to meet criteria than did older dogs. However, no age effect was found on extinction for either the number or duration of trials (P > 0.05 for both), implying that persistence in previously reinforced behaviour did not influence the age sensitivity found in task acquisition. Overall, these results suggest that, whereas the clicker may prolong behaviour without primary reinforcement, it does not reduce the training time of a simple operant task in dogs when primary reinforcement is briefly delayed. We speculate that the clicker may be most useful in maintaining established behaviours when primary reinforcement is unavailable or when its delivery is impractical. Additionally, we found that basenji dogs may become progressively impaired with age in the acquisition of stimulus-reward contingencies.     

Elevated temperature, per se, does not limit the ability of rainbow trout to increase stroke volume

Key questions remain about the regulation and limits of cardiac function in fish challenged with elevated temperature, and to what extent sex differences influence cardiac performance. In this study, we investigated the in vivo relationship between heart rate (f_H), stroke volume (S_V), and cardiac output (Q) in quiescent, sexually immature rainbow trout (Oncorhynchus mykiss Walbaum) when challenged with: (1) an acute increase in water temperature from 14 to 24 °C at 2 °C h⁻¹ and (2) a 50% reduction in f_H at 24 °C, achieved through the incremental administration of zatebradine hydrochloride (total dose 2 mg kg body mass⁻¹). There were no statistically significant (P<0.05) sex differences in cardiovascular function as temperature was raised to 24 °C. In males (N=10) and females (N=9), f_H increased in a linear fashion with water temperature (from ∼60 beats min⁻¹ at 14 °C to ∼125 beats min⁻¹ at 24 °C; Q₁₀=2.1), S_V was largely unchanged, and systemic blood pressure (P_DA) increased only slightly (by approx. 0.5 kPa) because the potential effect of increased Q on P_DA was mostly offset by a 35% decrease in systemic vascular resistance (R_sys). At 24 °C, zatebradine treatment halved f_H in both sexes, and yet Q was maintained at pre-treatment levels due to a doubling of S_V. Overall, these results: (1) indicate that the in vivo cardiovascular response of quiescent, immature, male and female trout to elevated temperature is similar and (2) challenge the current dogma about how temperature affects cardiac function in fishes. Specifically, unlike previous in vitro or in situ studies, our data demonstrate that fish are capable of maintaining or even increasing S_V at high temperatures. This suggests that aspects of cardiac control favor an increase in f_H as temperatures rise, or that increases in cardiac output to meet the fish’s metabolic demands at high temperatures are met solely through an increase in f_H because tachycardia is a requisite (unavoidable) physiological response.     

Melatonin does not affect the black pigment migration in the crab Neohelice granulata

N-acetyl-5-methoxytryptamine or melatonin is a multifunctional molecule. The main physiological function, at least in vertebrates, is to transduce to the animal the photoperiodic information and regulate rhythmic parameters. But studies have also observed the action of this molecule on pigment migration in ectothermic vertebrates. Thus the aim of this paper was to investigate in vivo and in vitro the influence of melatonin on the pigment migration in melanophores of the crab Neohelice granulate. Injections of melatonin (2 × 10⁻⁹ moles · crab⁻¹) at 07:00 h or 19:00 h did not affect (p > 0.05) the circadian pigment migration of the melanophores in constant darkness. Additionally no significant pigment migration (p > 0.05) was verified in normal and eyestalkless crabs injected with melatonin (10⁻¹⁰–10⁻⁷ moles · crab⁻¹) during the day or night. In the ^{in vitro} assay, the response of melanophores to the pigment-dispersing hormone in eyestalkless crabs injected with melatonin (2 × 10⁻⁹ moles · crab⁻¹) 1 and 12 hours before the observations did not differ (p > 0.05) from the control group (injected with physiological solution). These results suggest that melatonin does not act as a signaling factor for pigment dispersion or aggregation in the melanophores of N. Granulate.     

Ring-like nucleoid does not play a key role in radioresistance of Deinococcus radiodurans

The conclusion based on transmission electron microscopy, “the tightly packed ring-like nucleoid of the Deinococcus radiodurans R1 is a key to radioresistance”, has instigated lots of debates. In this study, according to the previous research of Pprl’s crucial role in radioresistance of D. radiodurans, we have attempted to examine and compare the nucleoid morphology differences among wild-type D. radiodurans R1 strain, pprf function-deficient mutant (YR1), and pprl function-complementary strains (YR1001, YR1002, and YR1004) before and after exposure to ionizing irradiation. Fluorescence microscopy images indicate: (1) the majority of nucleoid structures in radioresistant strain R1 cells exhibit the tightly packed ring-like morphology, while the pprl function-deficient mutant YR1 cells carrying predominate ring-like structure represent high sensitivity to irradiation; (2) as an extreme radioresistant strain similar to wild-type R1, pprl completely function-complementary strain YR1001 almost displays the loose and irregular nucleoid morphologies. On the other hand, another radioresistant pprl partly function-complementary strain YR1002’s nucleiods exhibit about 60% ring-like structure; (3) a Pprl C-terminal deletion strain YR1004 consisting of approximately 60% of ring-like nucleoid is very sensitive to radiation. Therefore, our present experiments do not support the conclusion that the ring-like nucleoid of D. radiodurans does play a key role in radioresistance. Supported by the National Basic Research Program of China (Grant No. 2004CB19604), the National Natural Science Foundation of China (Grant No. 30330020), and the National Science fund for Distinguished Young Scholars (Grant No. 30425038)     

Unfolding of tertiary structure ofHalobacterium halobium flagellins does not result in flagella destruction

The structure ofHalobacterium halobium R₁M₁ flagella is investigated by the methods of scanning microcalorimetry, circular dichroism, and electron microscopy. It is shown that melting curves of flagella in solutions with a different concentration of NaCl display only one peak of heat capacity that corresponds to one cooperatively melting domain. It is found that flagella do not dissociate after melting. The possible structural organization of archaebacterial flagella is discussed.     

Pretreatment with heat does not affect double-strand breaks DNA rejoining in Chlamydomonas reinhardtii

We aimed to clarify if heat pretreatment could protect Chlamydomonas reinhardtii cells from gamma rays DNA damaging action. It was studied whether: (1) heat pretreatment could accelerate DNA DSB rejoining; (2) chloroplast chaperones (HSP70B, HSP90C) could be involved in protection from radiation-induced DNA DSB.It was obtained that heat pretreatment (37–42 °C) induced minor DNA DSB levels which might be insufficient as signals for DNA DSB repair induction. No correlation between chaperones overproduction and DNA DSB rejoining was shown. These are probably the first data that HSP70B and HSP90C do not protect DNA against radiation-induced damage in a plant model system.     

The Asc locus for resistance to Alternaria stem canker in tomato does not encode the enzyme aspartate carbamoyltransferase

The fungal disease resistance locus Alternaria stem canker (Asc) in tomato has been suggested to encode the enzyme aspartate carbamoyltransferase (AC Tase). To test this hypothesis a segment of the tomato ACTase gene was amplified by the polymerase chain reaction (PCR) using degenerate primers. The PCR product obtained was subsequently used to isolate an ACTase cDNA clone. Restriction fragment length polymorphism (RFLP) linkage analysis showed that the ACTase gene and the Asc locus do not cosegregate. RFLP mapping positioned the ACTase gene on chromosome 11, while the Asc locus is located on chromosome 3. These results exclude the possibility that the ACTase protein is encoded by the Asc locus.     

Directed inactivation of the psbl gene does not affect Photosystem II in the cyanobacterium Synechocystis sp. PCC 6803

PsbI is a small, integral membrane protein component of photosystem II (PSII), a pigment-protein complex in cyanobacteria, algae and higher plants. To understand the function of this protein, we have isolated the psbI gene from the unicellular cyanobacterium Synechocystis sp. PCC 6803 and determined its nucleotide sequence. Using an antibiotic-resistance cartridge to disrupt and replace the psbI gene, we have created mutants of Synechocystis 6803 that lack the PsbI protein. Analysis of these mutants revealed that absence of the PsbI protein results in a 25–30% loss of PSII activity. However, other PSII polypeptides are present in near wild-type amounts, indicating that no significant destabilization of the PSII complex has occurred. These results contrast with recently reported data indicating that PsbI-deficient mutants of the eukaryotic alga Chlamydomonas reinhardtii are highly light-sensitive and have a significantly lower (80–90%) titer of the PSII complex. In Synechocystis 6803, PsbI-deficient cells appear to be slightly more photosensitive than wild-type cells, suggesting that this protein, while not essential for PSII biogenesis or function, plays a role in the optimization of PSII activity.     

Prey envenomation does not improve digestive performance in Taiwanese pit vipers (Trimeresurus gracilis and T. stejnegeri stejnegeri)

It has been a common belief that snake venom may help in the digestion of its prey, although direct examples and supporting evidence have not been sufficient. To address this, the present study examined whether pre-injecting natural amounts of pit viper venom into experimental mice may accelerate their digestion by the snakes or gain energy benefit as compared to the control without the envenomation. Live adults of two Asian pit viper species Trimeresurus gracilis and T. stejnegeri stejnegeri, which inhabit the cold and warm environment respectively, were the subjects studied herein. A natural dose of 1.2 mg of each of the pit viper venom in phosphate-buffered saline (PBS) was injected into the mouse (about 10% of the snake mass) before it was being fed to the same species of vipers, while the pit vipers in control group were given mouse injected with sterile PBS. The snakes were kept at 14 °C or 24 °C, and parameters of gut passage time, costs of digestion, and/or digestive efficiency were measured. The results did not support the hypotheses that envenomation facilitates prey digestion. The venom in fact caused longer first defecation time and lower assimilation energy at 14 °C. Besides, the time to reach the oxygen consumption peak, and the first defecation time of T. s. stejnegeri were longer than that of T. gracilis.     

Life History of the Whitefly Predator Nephaspis oculatus (Coleoptera: Coccinellidae) at Six Constant Temperatures

Nephaspis oculatus (Blatchley) is a predator of whiteflies including Bemisia tabaci (Gennadius) and B. argentifolii Bellows and Perring. The usefulness of this and other coccinellid predators of whiteflies could be improved by better information on responses to different temperatures experienced in the field. We reared N. oculatus from egg to adult in incubators at six constant temperatures and observed a linear relationship between developmental time and temperature for all preimaginal lifestages through all but the highest temperatures (33°C). Four larval instars were observed, although a significant proportion of third instar larvae molted directly to the pupal stage at 29°C and above. Complete life table data were obtained at all temperatures except 33°C. Survivorship was greatest over all immature stages at 26°C and finite and intrinsic rates of increase were highest at this temperature. Although 26°C appeared to be the optimum temperature for population growth, estimated rates of increase at all temperatures tested above 20°C were within 75% of the maximum, indicating that the beetle can perform well within the temperature range usually found in greenhouses.     

Energetics and litter size variation in domestic dog Canis familiaris breeds of two sizes

We measured resting metabolic rate (RMR), daily energy expenditure (DEE) and metabolisable energy intake (MEI) in two breeds of dog during peak lactation to test whether litter size differences were a likely consequence of allometric variation in energetics. RMR of Labrador retrievers (30 kg, n=12) and miniature Schnauzers (6 kg, n=4) averaged 3437 and 1062 kJ/day, respectively. DEE of Labradors (n=6) and Schnauzers (n=4) averaged 9808 and 2619 kJ/day, respectively. MEI of Labradors (n=12) was 22448 kJ/day and of Schnauzers (n=7) was 5382 kJ/day. DEE of Labrador pups (2.13 kg, n=19) was 974 kJ/day and Schnauzers (0.89 kg, n=7) were 490 kJ/day. Although Labradors had higher MEIs than Schnauzers during peak lactation, there was no difference in mass-specific energy expenditure between the two breeds. Hence, it is unlikely that litter size variation is a likely consequence of differences in maternal energy expenditure. Individual offspring were relatively more costly for mothers of the smaller breed to produce. Therefore, litter size variations were consistent with the expectation that smaller offspring should be more costly for mothers, but not that smaller mothers should per se invest more resources in reproduction.     

Interaction of the autonomic nervous system with intrinsic cardiac rate regulation in the guinea-pig, Cavia porcellus

In the denervated mammalian heart a change in right atrial pressure will still alter heart rate (intrinsic rate response, IRR). We have examined the IRR in isolated right atria of the guinea-pig maintained in oxygenated Krebs–Henseleit solution at 37°C, to compare with and extend studies in other species, and to determine whether the guinea-pig is a suitable model for electrophysiological studies of the IRR. Baseline diastolic transmural pressure was set at 2 mmHg. A 6-mmHg increase in right atrial pressure (RAP) caused an increase in atrial rate that reached a steady value of 15 min⁻¹ after 1–2 min. This response was enhanced by carbamylcholine and attenuated by isoprenaline. The influence of RAP on the rate response to vagal stimulation was examined. With RAP set at 8 mmHg, the reduction in atrial rate following vagal stimulation was 72±5% of that at 2 mmHg (n=6, mean±S.E., P<0.005). Continuous vagal stimulation produced a sustained bradycardia, and the effect of this bradycardia on the IRR was examined. When atrial rate was reduced 6% by vagal stimulation, the IRR was augmented to 202±21% of the control (n=6, P<0.005). This augmentation was larger (P<0.05) than that seen when atrial rate was reduced 8% by carbamylcholine (130±8% of control; n=7, P<0.05). Overall, the IRR in the guinea-pig is similar to that in the rabbit, and shows similar interactions with the autonomic nervous system.